Odd–even effects in the director dynamics of the nematic phases formed by symmetric and non-symmetric liquid crystal dimers. An ESR investigation

Abstract

Liquid crystal dimers, in which two mesogenic groups are linked by a flexible spacer, have been found to exhibit a pronounced odd–even effect in many of their physical properties. These include the nematic–isotropic transition temperatures, transitional entropies and their orientational order. In contrast, there have been far fewer studies of the director dynamics within the nematic phases of liquid crystal dimers. Here we present results from investigations of the magnetic field-induced relaxation time for the director in the nematic phase, using ESR spectroscopy, for the non-symmetric α-(4′-cyanobiphenyl-4-yloxy)-ω-(4-alkylanilinebenzylidene-4′-oxy)alkane dimers and the symmetric α,ω-bis(4-alkylanilinebenzylidene-4′-oxy)alkane dimers. Unlike quantities such as the orientational order parameter it is not so apparent as to what temperature scale should be used to compare the relaxation time, which depends on the rotational viscosity coefficient γ₁. We discuss various possibilities and show that for a certain choice the relaxation times do exhibit an odd–even effect as well as a dependence on the nature of the smectic phase following the nematic.